Most ceramics are prone to fatigue, a form of stress-aided corrosion in which small flaws present in the material increase slowly in size until they become large enough to cause structural failure at stress levels only a small fraction of the short-term fracture stress. Essential for fatigue failure to occur is the simultaneous presence of stress and water, factors both present in the oral environment, Judging both by published studies which indicate unexpected clinical failures are a problem associated with coramic restorations and by the large number of in vivo porcelain repair materials available commercially, fatigue is likely to be a factor in limiting the lifetime of a ceramic restoration. Although ceramics have long been used in dental restorations, fundamental fatigue data are lacking which are essential for assessing the role played by fatigue in the long-term clinical performance of ceramic restorative materials. Published data exist to indicate that the strength of dental porcelains is lower in water than in air, but there has been no study found which reports information relating to the basic fatigue process, the slow growth of cracks upon imposition of an applied stress. No work, not even limited strength studies, could be found of how the oral environment affects the mechanical and fatigue behavior of dental ceramics. The objective of this proposed research is to measure the fundamental parameters which determine the rate of slow crack growth and thus define the susceptibility of a material to fatigue failure. Included in the study will be fedspathic and aluminous porcelains, and hopefully, the new ceramic core materials. Standard fracture mechanics tests, such as the double torsion and stress rupture techniques, will be performed in both water and in a simulated oral environment using synthetic saliva at 37 C. Emphasis will be given in the tests to determine whether an endurance limit, or threshold level of stress intensity factor, must be exceeded before crack growth can be initiated. The crack growth parameters will be used to construct lifetime prediction diagrams so that estimates can be made of the expected survival time of the different ceramic restorative materials under realistic fatigue conditions in an oral environment.